This application is based on and claims the benefit of German Patent Application No. 198 33 549.0 filed Jul. 25, 1998, which is incorporated by reference herein.
This invention relates to a receiver for use in a transmission system for spectral-coded data, and a method for reception of such spectrally coded data.
The prior art, such as Patent Application DE 197 23 103.9, for example, discloses transmission methods that employ spectral coding. Such a transmission network consists of optical transmission lines and optical splitters, as well as optical amplifiers if necessary, and is used for the transmission of coded, multiplexed optical signals. Each transmitter contains a coder in which the signals to be transmitted are coded before they are dispatched into the optical transmission network. The coding is done optically, e.g. by frequency coding using an optical filter. Each receiver that wants to receive the data from a special transmitter must contain a decoder that is synchronized to the encoder of this specific transmitter. In the simplest case, the frequency ranges that are permeable for optical signals and the frequency ranges that are blocked for optical signals are the same in the coder and in the decoder. This method is known by the term CDMA (Code Division Multiple Access). The senders used in this system can be light-emitting diodes, for example, the broad-band emission spectrum of which runs through an optical filter. The optical filter can be a Fabry-Perot filter, for example, that converts the broad-band spectrum in a frequency polarization comb assembly.
On the receiving side, Patent Application DE 19723103.9, for example, discloses a differential receiver. Such a receiver contains a decoder that is synchronized to the coder of the transmitter, the optical signals of which it wants to or is authorized to receive. In particular, the simultaneous activity of a plurality of transmitters in the optical transmission leads to side-to-side crosstalk in the receiver of the optical signals from the transmitters. The signals to be detected, therefore, also contain portions of optical signals from other transmitters which are experienced as interference. The prior art discloses ways to suppress this interference by compensation in the differential receiver. Such a receiver with compensation of the two signal branches is disclosed, for example, in Patent Application DE 19748756.4.
In the optical differential receiver for frequency-coded optical CDMA systems disclosed in this patent, it is necessary, within the electrical modulation bandwidth, e.g., 108 MHz for a nxc2x7155 MBit/s system, to guarantee the synchronization of the two receiver arms with regard to the amplitude and phase shifts of the signals of less than 0.1 dB and  less than 10 ps for all frequencies. It is thereby possible to achieve a bit error rate of  less than 10xe2x88x929, for example. The requirements for such an optical differential amplifier, however, increase with the number of simultaneous transmitters in the network, so that it becomes increasingly more difficult to connect much more than 8 transmitters to the network. For large numbers of transmitters, therefore, the concept of the differential receiver must be discarded. Nevertheless, the receiver claimed by the invention can be used to achieve the necessary suppression of the side-to-side crosstalk of the channels that are not to be received.
The receiver of the invention, an input signal can be evaluated that is modulated on the transmitter or receiver side by an impressed jitter of the carrier frequency of the frequency comb of by the jitter of the frequency intervals of the frequency comb.
In accordance with a further aspect of the invention, the filter characteristic of the at least one optical filter (3) can be modulated at a frequency f0 and the electrical signal obtained by means of the light-sensitive detector (4) is applied to a microwave detector (6).
An advantage of this aspect of the invention over similar devices of the prior art is that a high number of transmitters can be operated in a transmission system. The sensitivity to crosstalk is largely eliminated by modulation of the optical signal.
It is advantageous if, in the receiver, a high suppression of the side-to-side crosstalk of the other channels is guaranteed by a modulation of the filter characteristic of the optical filter. The modulation of the optical filter is advantageously performed at a high frequency, and the modulated signal is evaluated with a microwave detector.
In one advantageous embodiment, the optical decoder is a Mach-Zehnder filter. Another embodiment uses a Fabry-Perot filter as the optical decoder. The optical filter is advantageously connected directly with a microwave generator. The modulation frequency is thereby always greater than the frequency of the bit rate of the data transmission.
Advantageous refinements and improvements of the receiver will be apparent from the detailed description below and the appended claims.
In accordance with a further aspect of the invention, a method for the reception of spectral-coded optical signals in a CDMA transmission system is provided whereby an optical signal is decoded, converted into an electrical signal and amplified, wherein the optical signal is modulated by modulation of the synchronization of the optical decoder at a first frequency, the modulated signal is converted in a light-sensitive detector into an electrical signal of a second frequency, and this electrical signal is transformed back into the base band in a detector.
In accordance with a yet further aspect of the invention, a method for the reception of spectral-coded optical signals in a CDMA transmission system is provided wherein an optical signal is decoded, converted into an electrical signal and amplified, wherein the optical signal is modulated by modulation in the transmitter, the modulated signal is converted in a light-sensitive detector into an electrical signal of a second frequency, and this electrical signal is transformed back into the base band in a detector.